Stereoselective syntheses of the marine diterpene dictyolene (20) and its C-11 epimer have been effected. The marine cembranolide crassin acetate (2, p. 7) has been converted to several methylene lactones of interest as possible antitumor agents. The pseudoguaianolides damsin (1) and tenulin (14) were employed as model compounds to study the introduction of a gamma-hydroxyl grouping into a fused ring gamma-butyrolactone. Synthetic approaches to pseudoguaianolides via epoxide rearrangement and to germacranolides via boronate fragmentation were pursued.